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HARDWICKE'S SCIENCE-GOSSIP. 



the structure of the ovum, so far as it differs from that 

 of other animals. 



The pigment is deposited in a thick uniform layer 

 in the outer membrane, orecto-sac. It is dark brown 

 in colour, and covers all the surface, except one small 

 round spot. 



Instead of being enclosed iri an opaque shell of 

 calcareous or horny matter, which is almost in- 

 variably the case with the ova of other animals, those 

 of the frog are surrounded by a fine transparent 

 homogeneous membrane. Between this membrane 

 and the ecto-sac clear gelatinous matter occupies the 

 space where the albumen of the egg is usually found. 

 It is quite different in its chemical composition from 

 the albumen of eggs, being allied to mucin. I have 

 not found any other animal secretion having exactly 

 the same chemical composition. Its great bulk in 

 proportion to the size of the ovum it surrounds is very 

 peculiar. It swells out to many times its original 

 bulk when it is deposited in water. It is usually 

 stated that it is simply for nourishing the embryo 

 during the process of development, but only a fraction 

 of its bulk is consumed when the young tadpoles 

 escape. It is generally supposed that the tadpoles 

 feed on it for some time after they are hatched, but 

 this is by no means invariably the case ; for large 

 masses of it are very common along the edges of 

 ponds and ditches after the young have escaped and 

 attached themselves to the stems and leaves of 

 aquatic plants. To satisfy myself that it was not 

 necessary to the young after they were hatched, I 

 removed about fifty of them as soon as they emerged 

 from the mass and placed them in a separate vessel. 

 They developed as rapidly and seemed as strong and 

 healthy as those I left in the vessel with the gelatinous 

 substance. There is no doubt but that this large 

 amount of gelatinous matter is intended to protect 

 the ovum from the extremes of temperature to which 

 it is exposed in the early spring. It can scarcely be 

 called highly nutritious, for I evaporated three ounces 

 of it over a hot-water bath, and obtained only seven 

 grains of solid matter, or only about one-half per 

 cent, the rest consisting of water. 



In addition to their transparency, the bulk, consist- 

 ence, and form of the envelopes, aid in transmitting 

 light in a rather curious, although accidental, manner. 

 Thus the ray of light {a, n, fig. 73) falling perpen- 

 dicularly on the surface of the spherical mass, passes 

 straight through to the ovum d, without its direction 

 being changed ; while the rays b, n, and c, n, falling 

 obliquely on the surface, are refracted towards the 

 axis and meet at d. The gelatinous mass, in fact, 

 acts as a lens, condensing all the rays of light which 

 strike its surface, so that they fall on the pigmented 

 ovisac, for we know that in a spherical lens the focus 

 is at the distance of the radius of the sphere. 



We have, in short, exhibited in the spawn of frogs, 

 mechanical arrangement somewhat similar to that 

 observed in the eye ; a dark pigmented layer for ' 



receiving the light, like the choroid coat ; a spherical 

 lens in front resembling the cornea; and an inter- 

 mediate, clear refracting medium corresponding to 

 the humours of the eye. 



Early in March 1878, I obtained a mass of frog's 

 spawn and divided it into three equal parts, each of 

 which I placed in a separate flat glass vessel. One of 

 these I placed in a window having a south-westerly 

 aspect, so as to be exposed to direct sunlight during the 

 greater part of the day. A second portion was placed 

 in the same window, but enclosed in a small dark 

 chamber, arranged so as to allow thorough ventilation. 

 The third portion was placed in a window looking 

 almost due north in the same room. The develop- 

 ment of the two portions in the south-western window 

 took place with equal rapidity, and the young tadpoles 

 escaped from both masses on the same day. The 

 eggs exposed to the light in the northern window 

 were much slower in their development, and were 

 hatched about a week later. The temperature in this 

 window was always several degrees lower than in the 

 other during the day ; occasionally the difference was 

 as much as 10° or 15°. A thermometer suspended 

 in the dark chamber, showed that the temperature in 

 it was as high as in the direct sunlight. These experi- 

 ments — which I have since repeated with the same 

 results — prove that heat is the sole, or at any rate the 

 principal agent for maturing the ovum, and that the 

 withdrawal of the stimulus of light does not check its 

 development. This is in accordance with what is 

 observed in nature ; the period of hatching varying 

 from a few days to several weeks, according to the 

 temperature of the atmosphere, clear, sunshiny frosty 

 weather retarding development as much as dull cold 

 weather. 



If our experiment stopped at this point, it would 

 seem as if this elaborate arrangement of pigment 

 layers and transparent envelopes had no reference 

 to the action of light. But if we watch the further 

 development of the tadpoles under the same conditions, 

 a remarkable difference between those exposed to 

 the light and those kept in the dark is soon manifest. 

 The former increase in size rapidly, and in due 

 time undergo metamorphosis, while the latter never 

 acquire full growth, are much paler than natural, and 

 their metamorphosis is retarded for an indefinite 

 period. I have kept tadpoles in an ordinary sitting- 

 room for about six months, removed from the window 

 as far as possible, and at the end of that period they 

 were only half grown, and showed no signs of meta- 

 morphosis. These experiments then demonstrate that 

 the absence of light does not prevent the hatching of 

 frog's spawn. Like the seed of plants, and the ova 

 of all other animals, a certain amount of heat is all 

 that is necessary. 



Light, however, appears to be necessary for the 

 development of healthy young ; and its influence 

 is very marked after the tadpoles are hatched. The 

 pigment is then enclosed in stellate cells. Under 



